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Ktnding Stress-Act ion.                       427

W causes a bending moment or turning effect round A equal
to W x /, and also exerts a downward pull to be balanced by
weight Wl;so that\V« \\\. The latter is called the shearing
force, and h felt on every vertical section of the beam. Wz and
W really form a couple with the arm /, and this can only be
balanced by another couple =(/ or t) x A n, a tension being felt
in the tipper fibres and a compression in the lower ones, shewn
respectively by the link A and strut B.

The case we have examined is that of a cantilever or overhung

Fig. 382  shews a supported beam  or girder, and the

action is here reversed, the lower fibres being in tension

the upper ft* compression.   Taking a bar of indiarubber, and

both before and after bending, it will be found that

€ is              shortened, t lengthened, while n is unaltered; * is

termed the neutral line or axis of the bar.
Position of Neutral Axis.—The bar "in Fig. $83 is beat
10 an            circle, and has A B far neutral axis, with fibres B c

€ G